I've been trying the easy East Prussia 1914 scenario and noticed that in all games, the other side, no matter who I play, always loses with negative thousands of victory points. Is this deliberate to make the scenario easy to play, or is it a bug?

I am pretty sure this just means you are playing well as both factions. You can up the difficulty and see if you get the same results.Another option is to play both sides and see which one you do best with if there is no AI.